A conventional mobile phone is used for a single user and, consequently, the mobile phone has a single microphone for receiving sound waves of human voice of the user of the phone and a single loudspeaker for reproducing sound for the user.
However, in some situations it is desirable to set up a call with more than two participants. Some prior art mobile phones, like the Ericsson mobile phone T28 and others, solve this problem by providing a conference call service that enables calls involving more than two participants. To create a conference call with three participants A, B, and C, A has a conventional prior art mobile phone with conference call capability and B and C may have either conventional prior art mobile phones or just conventional telephone sets. A makes a call to B. When B answers the call, A puts B on hold and makes a second call, this time to C. Then, A joins the calls through a particular function provided in the network. Now, all participants can hear each other at the same time.
A conference call can be created in other ways than the example described above, but this is well known and therefore not described herein in detail.
When for example A and B are at the same place and want to set up a conference call with C who is remote, it is inconvenient to create a conference call involving three phones as described above. Further, it seems to be unnecessarily expensive to have two ongoing calls at the same time. A more common and less expensive way of setting up a conference call in this case is to use a system phone provided with an additional fixed loudspeaker and fixed microphone. Then, A can make a call to C and instead of using the regular handset both A and B speak with C through the additional microphone and listen to C via the additional loudspeaker. Thus, only one call is charged at the same time.
However, it is not always possible to find a system phone in the vicinity. For example in an airport transit hall or another public place you are reduced to the case with two mobile phones, one for A and one for B for creating a conference call with the third participant C.